Subtasks together with task descriptions are the best way to add those necessary details that will help a user get the task done. You can create new subtasks in the Full task details view. When you create or edit a task you have the option to add subtasks and as you complete those subtasks you can check them off.
If you would like to edit subtask names, you need to click on them and edit inline. In order to delete a subtask, you need to hover over the subtask and click on the trashcan icon.
In order to tell if a task has subtasks, you can go to the Full Tasks Details view of the task or use the Simple or Board view. You will see a subtask icon and how many of the subtasks are completed compared to the total number of subtasks.
Note: You can't track time for subtasks or assign them to users, this is still done on a task level.